Field of the Invention
The disclosure relates generally to a synchronous rectifier controller, and more particularly it relates to a synchronous rectifier controller for providing adaptive dead time compensation for a synchronous rectifier under adaptive output powers.
Description of the Related Art
FIG. 1 shows a flyback converter. The primary side of the flyback converter includes a primary winding LM and a power switch Q1 which are connected in series. By controlling the switching of the power switch Q1, the energy at the primary side can be transferred to the secondary side of the flyback converter in a flyback manner.
The secondary side includes a secondary winding (inductor) LS, a synchronous rectifier, and a synchronous rectifier controller (SR controller, also referred to as “secondary-side controller”) 10. The synchronous rectifier includes a switch Q2 which can be implemented by a MOSFET. The SR controller 10 is coupled to control the switch Q2. For adaptive output powers, the output voltage VOUT of the flyback converter is variable according to commands sent by a device (such as a smartphone or tablet) that is being charged by the flyback converter, and the discharging slope of the inductor (magnetic device) LS changes with the variation of the output voltage VOUT. In the prior art, the drain-source voltage (VDS) of the switch Q2 affected by the discharging of the indictor LS is sensed and compared with a synchronous rectifier turn-off threshold (SR turn-off threshold) to control the turned-off state of the switching of the switch Q2. However, the SR turn-off threshold is a fixed value. Thus, for different inductor discharging slopes, the dead time of the synchronous rectifier varies with the output voltage VOUT. In such cases, the dead time would be longer at the lower output voltage conditions.